"Any even moderately complex app is going to leverage database-specific performance enhancements that can't be abstracted away."
I'm not sure I totally agree with you here; I run what I would consider to be a "moderately complex" app (basically an e-commerce site) using PHP and MySQL with no data abstraction layer (if I had my choice, I WOULD use one...). We don't leverage any database-specific performance enhancements directly via our queries (we do use indexes and the like on particular tables to help speed up certain queries though), because MySQL is generally better at it than I would be. Granted, I'm no DBA, and I can see how some people out there really CAN tweak their own queries to be more efficient than the database system optimization might be. But in general, in the ideal situation, I'd much rather have clean, portable code, and once in awhile have to avoid the abstraction layer (any good database abstraction layer will allow you to write your own SQL if you wish, and if it doesn't, there's nothing stopping you from not using the abstraction layer for that particular query) then to not use the abstraction layer at all.
I guess my point is, in general, you don't start out optimizing; you optimize where you need to optimize when you need to, and the benefits of using an abstraction layer for 90% of the stuff I would do far outweigh any possible downsides.
I'm not sure I totally agree with you here; I run what I would consider to be a "moderately complex" app (basically an e-commerce site) using PHP and MySQL with no data abstraction layer (if I had my choice, I WOULD use one...). We don't leverage any database-specific performance enhancements directly via our queries (we do use indexes and the like on particular tables to help speed up certain queries though), because MySQL is generally better at it than I would be. Granted, I'm no DBA, and I can see how some people out there really CAN tweak their own queries to be more efficient than the database system optimization might be. But in general, in the ideal situation, I'd much rather have clean, portable code, and once in awhile have to avoid the abstraction layer (any good database abstraction layer will allow you to write your own SQL if you wish, and if it doesn't, there's nothing stopping you from not using the abstraction layer for that particular query) then to not use the abstraction layer at all.
I guess my point is, in general, you don't start out optimizing; you optimize where you need to optimize when you need to, and the benefits of using an abstraction layer for 90% of the stuff I would do far outweigh any possible downsides.